The use of membranes in various forms, such as sheets and hollow fibers, for the separation of fluids to obtain a desired portion of the fluid is well known. Generally, one or several components of the feed fluid permeate through the membrane and are collected as the permeate portion. The portion of the fluid that cannot pass through the membrane, the retentate, is retained and discharged as fresh portions of the fluid to be separated are supplied to the membrane.
Membrane separation modules have been of two different types, hollow fiber and sheet membrane. The modules of the present invention may be either of the former type or a combination of the two types. To date, the two most common configurations for sheet membrane modules have been of the plate and frame type or of the spiral wrap type.
In the plate and frame configuration, a sheet of membrane material is compressed between two rigid frames which hold it flat, provide support against the differential fluid pressure, and provide fluid flow ports to direct the fluid streams across the membrane surface. The plate and frame design requires a large number of components, with commensurate costs, per unit of membrane area. Sealing the frames against the membranes to achieve a tight seal without damaging the membrane is a problem.
U.S. Pat. No. 3,684,097, for instance, provides a plate and frame device for oxygenating or dialyzing blood which includes a pair of frames having rectangular openings and a gas permeable membrane on each face. A plurality of frames are stacked upon each other in substantial contact to define a thin blood film flow space therebetween.
U.S. Pat. No. 4,115,274 is directed toward a reverse osmosis desalinating apparatus which employs a series of porous discs, each covered on both sides by a membrane. The discs are compressed at their edges between pairs of module plates which are arranged in stacks. Alternate module plates in the stack are rotated through 180.degree. to provide a zig-zag flow path. Desalinated water enters the porous discs by reverse osmosis and flows radially outward for collection.
U.S. Pat. No. 4,735,718 is directed toward a multilayer membrane separator for the filtration of liquids. The separator provides at least two membrane units, each unit having first and second membrane sheet layers adhered to a filtrate spacer layer. First and second retentate spacers are in direct contact with first and second membranes, respectively and provide a plurality of channels for passage of filtrate and retentate.
In the spiral wrap configuration, a sheet of porous support material is enclosed is a long sleeve of sheet membrane. The sleeve is typically formed by folding a web of membrane over a web of porous support material then sealing the two edges of the membrane. The long section of sleeve is then rolled into a spiral so that a fluid to be treated can flow from one edge of the spiral to the other in the axial direction. The permeating fluid flows within the spiral sleeve and is collected by a permeate manifold at the center of the spiral. The spiral wrap design requires the permeate to flow around inside the spiral sleeve and through the porous support material for considerable distances. This results in performance penalties caused by a significant permeate pressure drop. The seal between the end of the membrane sleeve and the permeate discharge manifold is difficult to make reliably. The end of the membrane sleeve must be sealed and both the sleeve and the side seals are unprotected in the feed fluid stream. These sheet-to-sheet seals must remain tight for the life of the module. Membrane damage during the wrapping process and during subsequent shifting of the wrap are potential problems.
Membranes formed as hollow fibers or tubes are also used readily as they are inherently strong to resist filtration pressure, they provide high surface area to volume ratios and, they can be particularly arranged in various mechanical mountings. Conventional separation modules are configured as long cylinders with the hollow fibers arranged in an axial direction and terminated by plugs of potting material. One or both plugs are sliced to expose the open fiber ends and permit the flow of the permeate from the lumen of the tubular fiber.
In the existing devices, the fluid to be separated may be supplied to the outside of a fiber and the permeate may be collected from the lumen of the fiber. Alternatively, the fluid to be separated may be supplied to the lumen of the fiber and the permeate drained from outside of the fiber.
Hollow fiber membranes may be conveniently mounted in annular or similar frames or retainers having a continuous perimeter and an open central portion. The fibers are strung across the open central portion of the frame and the ends are embedded in the retainer thereby forming a wafer. The ends of the fibers are exposed at the outside surface of the retainer, giving access to the interior of the fibers, while the outside surfaces of the fibers are accessible in the open central portion of the retainer.
In order to obtain relatively high volume separation rates, membrane wafers are generally stacked coaxially so that the retainers bear on each other in sealing contact. Tight sealing of adjacent wafers is essential to avoid contamination of retentate and permeate. To achieve desirably high volume separation rates, the pressure of the fluid being subjected to the separation process is as large as possible. However, the fluid pressure is limited by the mechanical strength of the fibers, so that they are neither crushed nor ruptured, depending upon the direction of the pressure differential. The operating pressure is also limited by the fluid forces that tend to force the wafers apart, threatening the sealing engagement of adjacent wafer retainers.
Such membranes and corresponding separators incorporating the membranes are disclosed in various patents and application. U.S. Pat. No. 3,993,816, for instance, describes an apparatus in which the interiors of the hollow fibers are connected to the exterior of the container so that either fluid permeate flows out of the apparatus or, in the event of fluid exchange, a second fluid flows in through the interiors of the fibers.
U.S. Pat. No. 4,752,305 provides a hollow fiber device for separating fluids and a related method. The device employs a bundle of fibers, oriented axially of the housing. The fluid feedstream is fed through the center of the bundle by a distributor tube, one end of which is plugged. The permeate fluids flow out of each end, via outlets, while the retentate is drawn from another outlet.
French Pat. No. 2,222,134 discloses a module separator wherein the wafers comprise a fabric placed in a tube perpendicular to the direction of flow so that the openings of the hollow fibers are located around the periphery.
Japanese Pat. No. 56-28031 discloses a hollow fiber membrane separator consisting of a hollow fiber tube plate formed by use of a synthetic resin which fixes and integrates the ends of the hollow tubes, and a channel for fluid flow outside of the hollow tubes. The tube plate is disposed such that the flow direction of the fluid outside of the tubes is perpendicular to the longitudinal axis of the hollow fiber tubes.
Finally, in German Pat. No. 2,650,341 a hollow fiber wafer is disclosed for use in a separator in which the hollow fibers are arranged in planes with an essentially non-parallel arrangement, essentially perpendicualr to the flow direction of the substance to be separated.
While the aforementioned art discloses some of the ways in which sheets of semipermeable membranes or hollow fibers may be employed in a separator apparatus, it is nonetheless desirable to provide a novel wafer element providing a plurality of sheet membrane sleeves and/or hollow tube layers, as well as a separation module, employing a plurality of such wafers in which the seals between adjacent wafers in the stack are maintained. Furthermore, it is desirable that the apparatus respond to changes in the fluid inlet pressure by altering the compression forces applied to the wafer stack to aid the sealing engagement of adjacent wafer retainers.
Additionally, it is desirable to provide multi port modules having three, four, six or more ports employing a variety of different types of sheet membrane sleeves and/or hollow members including hollow fiber materials of different permeabilities, porous tube members supporting semipermeable membrane materials and non-porous tubes useful for carrying heat exchange medium. Such modules have utility in a variety of methods which can be employed for separations, additions to the feedstream and reactions of feedstream components as will be explained in greater detail hereinbelow.